Ultraviolet (UV) lamp systems are commonly used for heating and curing materials such as adhesives, sealants, inks, and coatings. Certain ultraviolet lamp systems have electrodeless light sources and operate by exciting an electrodeless plasma lamp with microwave energy. In an electrodeless ultraviolet lamp system that relies upon excitation with microwave energy, the electrodeless plasma lamp is mounted within a metallic microwave cavity or chamber. One or more microwave generators, such as magnetrons, are coupled via waveguides with the interior of the microwave chamber. The magnetrons supply microwave energy to initiate and sustain a plasma from a gas mixture enclosed in the plasma lamp. The plasma emits a characteristic spectrum of electromagnetic radiation strongly weighted with spectral lines or photons having ultraviolet and infrared wavelengths.
To irradiate a substrate, the ultraviolet light is directed from the microwave chamber through a chamber outlet to an external location. The chamber outlet is capable of blocking emission of microwave energy while allowing ultraviolet light to be transmitted outside the microwave chamber. A fine-meshed metal screen covers the chamber outlet of many conventional ultraviolet lamp systems. The openings in the metal screen transmit the ultraviolet light for irradiating a substrate positioned outside the RF chamber; yet substantially block the emission of microwave energy.
Some applications of the UV lamp systems require very precise intensities of ultraviolet light. These applications are sensitive to changes in the UV light intensity, requiring the light intensity to be substantially constant. Providing a substantially constant UV light intensity presents some challenges. First, manufacturing tolerances associated with the magnetrons play a significant role in the output of each magnetron, which is directly proportional to the intensity of the UV light. A manufacturer of a magnetron may have output target values for a magnetron of, for example, approximately 3,000 W. Due to manufacturing tolerances, however, the actual output value of the magnetron could be lower than or higher than the 3000 W target output. Some manufacturers supply tolerance ranges with their magnetrons, without providing specific output values of each magnetron. This introduces some uncertainty as to the actual output of the magnetron used in the UV lamp system. Variances in tolerances between the magnetrons leads to variances in the outputs of each of the magnetrons, which causes UV light intensities to be different between different lamp systems.
A second challenge to providing substantially constant light intensity from the UV lamp systems is the tendency for the output power to drop as the lamp system and magnetrons heat up. As the UV lamp system heats up, warming the ceramic magnets in the magnetrons, the strength of the magnets declines. Input voltage to the magnetron is a function of current and the magnetic field and as a result of a declining magnetic field, the input voltage to the magnetron also drops. Given a substantially constant current to the magnetron, the drop in input voltage results in a drop in input power. As input power to the magnetron is reduced, so to is the output power, reducing the intensity of the ultraviolet light. After about 5 to 10 minutes of operation, the thermal changes to the system stabilize which then settles the output power of the magnetrons and ultimately the ultraviolet light intensity.